Storlazzi 2008 Estuarine, Coastal and Shelf Science

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The relative contribution of processes driving variability in flow, shear,and turbidity over a fringing coral reef: West Maui, Hawaii

Curt D. Storlazzi*, Bruce E. Jaffe

U.S. Geological Survey, Pacific Science Center, 400 Natural Bridges Drive, Santa Cruz, CA 95060, USA

Received 17 October 2007; accepted 18 October 2007

Available online 30 October 2007

Abstract

High-frequency measurements of waves, currents and water column properties were made on a fringing coral reef off northwest Maui,

Hawaii, for 15 months between 2001 and 2003 to aid in understanding the processes governing flow and turbidity over a range of time scales

and their contributions to annual budgets. The summer months were characterized by consistent trade winds and small waves, and under these

conditions high-frequency internal bores were commonly observed, there was little net flow or turbidity over the fore reef, and over the reef flat

net flow was downwind and turbidity was high. When the trade winds waned or the wind direction deviated from the dominant trade wind

orientation, strong alongshore flows occurred into the typically dominant wind direction and lower turbidity was observed across the reef. During

the winter, when large storm waves impacted the study area, strong offshore flows and high turbidity occurred on the reef flat and over the fore

reef. Over the course of a year, trade wind conditions resulted in the greatest net transport of turbid water due to relatively strong currents, mod-

erate overall turbidity, and their frequent occurrence. Throughout the period of study, near-surface current directions over the fore reef varied on

average by more than 41 from those near the seafloor, and the orientation of the currents over the reef flat differed on average by more than 65

from those observed over the fore reef. This shear occurred over relatively short vertical (order of meters) and horizontal (order of hundreds of

meters) scales, causing material distributed throughout the water column, including the particles in suspension causing the turbidity (e.g. sed-

iment or larvae) and/or dissolved nutrients and contaminants, to be transported in different directions under constant oceanographic and mete-orologic forcing.

Published by Elsevier Ltd.

Keywords: coral reefs; waves; tides; currents; turbidity; shear; USA; Hawaii; Maui

1. Introduction

Coral reefs typically grow in relatively clear, oligotrophic

waters. Land use practices such as overgrazing and coastal

development can increase the supply of terrestrial sedimentto the nearshore. Fine-grained terrestrial sediment can increase

turbidity, which in turn, decreases light available for photosyn-

thesis and can create physiological stress or even coral mortal-

ity (Marszalek, 1981;Buddemeir and Hopley, 1988; Acevedo

et al., 1989; Fortes, 2000). The syntheses by Rogers (1990)

and Fabricius (2005) noted that the magnitude and duration

of sediment and turbidity play a significant factor in the eco-

logical response of both individual corals and coral reef

ecosystems.

Most coral reef sampling protocols (e.g. Devlin and Lourey,2000) and scientific studies investigating the influence of land-

based material on nearshore coral reefs have employed limited

sampling schemes over the course of a day(s) or tidal cycle(s);

these data are then often used by regulatory agencies (e.g.

State of Hawaii, 2004) to determine if specific water bodies

meet defined water quality standards. A number of recent stud-

ies, however, suggest that flow and water column properties

over a reef flat vary temporally (Ogston et al., 2004) and those

over a fore reef are spatially heterogeneous (Storlazzi et al.,

2006b). While a number of investigations have focused on

* Corresponding author.

E-mail addresses: [email protected](C.D. Storlazzi), [email protected]

(B.E. Jaffe).

0272-7714/$ - see front matter Published by Elsevier Ltd.

doi:10.1016/j.ecss.2007.10.012

Available online at www.sciencedirect.com

Estuarine, Coastal and Shelf Science 77 (2008) 549e564www.elsevier.com/locate/ecss
mailto:[email protected]:[email protected]://www.elsevier.com/locate/ecsshttp://www.elsevier.com/locate/ecssmailto:[email protected]:[email protected]


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wave-, wind- or tidally-driven flow and transport along or

across a reef (e.g. Roberts et al., 1980; Wolanski and

Delesalle, 1995; Kraines et al., 1998; Lugo-Fernandez et al.,

1998; Tartinville and Rancher, 2000; Leichter et al., 2003;

Storlazzi et al., 2004; Lowe et al., 2005), most of these inves-

tigations have been limited in duration to the course of a few

weeks to a few months and thus the contribution of theobserved mechanisms driving flow and transport to annual

budgets was not constrained.

This paper addresses the nature of flow and water column

properties on the fringing reef of Kahana, northwest Maui,

over a range of seasons. An experiment was designed with

the goal to better understand the controls on hydrographic var-

iability along a relatively geometrically-simple fringing reef

and determine the contribution of these different processes

to annual budgets. The observations described here elucidate

the complex interactions between the wind, waves, tides,

high-frequency motions, and lower-frequency currents that

drive significant spatial and temporal variations in flow and

turbidity off northwest Maui.

2. Study area

The island of Maui, Hawaii, USA, is located at 20.8 N,

156.5 W in the north-central Pacific between the islands of

Molokai, Lanai, and Hawaii (the Big Island) in the Hawai-

ian Archipelago (Fig. 1). The northwest Maui coastline is

characterized by a series of sandy beaches, fringing reefs

and a sandy insular shelf. Over the past two decades, a number

of factors have affected the quality of the nearshore waters off

northwest Maui. Coastal development and agriculture have

increased runoff and the supply of sediment to northwestMauis coastal waters, while terrestrial wastewater injection

has increased the volume of nutrients percolating out of the

shoreface via submarine groundwater discharge (Soicher

and Peterson, 1996; West Maui Watershed Management Pro-

ject, 1996; De Carlo and Dollar, 1997; Dollar and Andrews,

1997).

2.1. Geology

Maui is comprised of two large basaltic shield volcanoes

that formed in the last 2 million years (Clague and Dalrymple,

1989) and are separated by a flat isthmus. Western Maui is

roughly 30 km long in the north-south direction and on aver-

age 20 km wide in the east-west direction. Land use in the

study area was historically dominated by pineapple and sugar-

cane cultivation; more recently, however, urbanization and

development along the shoreline have increased substantially

(M&EPacific, 1991).

The shoreline in the study area is characterized by small

basaltic headlands, carbonate sand beaches and a few small,

ephemeral stream mouths that drain coastal development and

upland pineapple fields. The geomorphology of the inner shelf

(1400 m) West Maui shield volcano, most of the precipita-

tion (100e400 cm/year) falls on the northern face of the vol-

cano while the south and southwest sides of the volcano

receive less than 40 cm/year on average (Fletcher et al.,

2002). This causes a north-south gradient in both stream

flow and terrestrial sediment discharge into the study area,

with greater freshwater and sediment discharge to the north

of Kahana than to the south. Most of the lower portions of

the streams are intermittent in nature, leaving the stream

beds dry during most of the year with stream flow at the lower

elevations occurring only during periods of heavy rain, typi-

cally in winter months (Soicher and Peterson, 1996).

550 C.D. Storlazzi, B.E. Jaffe / Estuarine, Coastal and Shelf Science 77 (2008) 549e564


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3. Field experiment and methods

Two primary instruments were used to acquire data during

these field deployments. The first instrument was a 600 kHz

upward-looking acoustic Doppler current profiler (ADCP).

The second primary instrument employed was a downward-

looking 5 MHz acoustic Doppler current velocimeter (ADV).

A logger collected and stored data from the ADV and four

external sensors: a pressure sensor, a conductivity-temperature

sensor and two 880-nm optical backscatter sensors (OBS).

Both of these instruments were mounted on the REEFPROBE

tripod that was deployed in a sand patch along the 10 m

isobath on the fore reef off Kahana between November 2001

and February 2003 (Storlazzi and Jaffe, 2003). Nine months

into the 15-month experiment, the smaller MiniPROBE pack-

age was deployed in a 2 m deep hole on the reef flat along the

2 m isobath (total depthw4 m) 350 m inshore of the REEFP-

ROBE instrument package; it had a 600 kHz upward-looking

ADCP similar to the one on the deeper REEFPROBE tripod

and a self-logging 880-nm OBS.

The instrument packages were typically deployed for

90e100-day periods, as constrained by the power consump-

tion. The logger on the REEFPROBE tripod collected data

from all sensors each hour to determine water depth, currents,

Fig. 1. Maps showing the location of the instrument packages relative to the morphology of the study area. NOAA aerial image showing the terrestrial landscape

and the location of major drainages; the bathymetry was developed from SHOALS LiDAR and USGS multibeam data.

551C.D. Storlazzi, B.E. Jaffe / Estuarine, Coastal and Shelf Science 77 (2008) 549e564


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wave heights, wave periods and wave direction, along with

salinity, temperature, and acoustic backscatter. The upward

looking ADCPs mounted on the REEFPROBE tripod and

the MiniPROBE sampled every 4 min to measure water depth,

temperature, and vertical profiles of currents and acoustic

backscatter. Cross-shore and alongshore directions are defined

relative to the general orientation of the isobaths and shoreline,which trend 30e210; positive alongshore is defined upwind

to the northeast. SeeTable 1for information on the instrumen-

tation and sampling schemes. Meteorologic data were ob-

tained from the National Climate Data Centers (NCDC,

2004) Kahului Airport Station (ID# 22516), approximately

25 km to the east of the study area. Sediment tube traps,

with their openings 0.6 m above the bed (mab), were attached

to both instrument packages during the last deployment

(November 2002 to January 2003) to capture suspended sedi-

ment for analysis. The tube traps were 60 cm long and had an

internal diameter of 6.7 cm; a hexagonal baffle (cells 0.5 cm

diameter, 7.6 cm long) consisting of phenolic resin and treated

with anti-fouling paint was placed in the top of the trap toreduce turbulence and prevent occupation by fish or crusta-

ceans. The trap design and sample processing followed the

methodology ofBothner et al. (2006).

It is relatively simple to calculate suspended sediment con-

centration in units of mass per volume from single-wavelength

optical or acoustic measurements in homogeneous sedimen-

tary environments using sediment samples collected in the

field (e.g. Larcombe et al., 1995). These optical and acoustic

sensors, however, suffer from calibration changes when parti-

cle size and particle composition (e.g. color) vary. Laser In-

Situ Scattering and Transmissometry (LISST) data collected

in 2003 (J. Harney, personal communication) showed thatboth the composition and grain size of suspended sediment

varied both spatially and temporally in the study area, with

both white carbonate sands and reddish-brown terrestrial

mud being observed.

The mixed grain size environment off west Maui made it

impossible to accurately calculate suspended sediment

concentration is units of mass per volume and thus only

measurements of turbidity are presented here. The OBSs

were calibrated to Nephelometric Turbidity Units (NTUs)

using 5 (0, 10, 50, 100 and 200 NTU) formazin calibration

standards. These NTU values from the OBSs were then corre-

lated to co-located acoustic backscatter data recorded by the

ADV and ADCPs that had been corrected for signal strength

decay with distance from the transducers due to spreadingand absorption using the method outlined by Deines (1999).

This method has been used successfully for quantitative mea-

surements of suspended sediment concentrations in numerous

studies (e.g.Thorne et al., 1991; Osborne et al., 1994; Reichel

and Nachtnebel, 1994; Holdaway et al., 1999). The correla-

tions between the ADCPs and ADVs corrected acoustic

backscatter and the co-located, unfouled OBS data had r2 cor-

relations of 0.64 (n 3128) and 0.81 (n 1051), respectively;

these correlations are significant above the 0.1% level. We

define turbidity flux as the product of the flow velocity (m/s)

and turbidity (NTU) and utilize it as a proxy for the physical

transport of particles that create optical backscatter and thus

turbidity. Because the turbidity/mass ratio is greater for finerparticles, and the ratio of fine to coarse particles in suspension

varies spatially and temporally, it is not possible to directly in-

fer mass flux from our measurements. Please seeStorlazzi and

Jaffe (2003)for more details on instrumentation, data acquisi-

tion and processing methodology.

4. Results

Data were acquired on 429 days during the 15-month pe-

riod between December 5, 2001 and 27 February, 2003; this

was more than 96% data coverage over the entire experiment.

The results are presented to address variability at two relativetime scales: (a) variability between seasons; and (b) variability

within seasons.

4.1. Summer

The summereearly fall regime is dominated by the north-

ernmost position of the Pacific High, which brings the heart

Table 1

Instrumentation and sampling schemes during the experiment

Instrument Depth Instruments Elevation Sampling rate Burst length Burst interval

(m) (m) (s) (s) (s)REEFPROBE 10 Acoustic Doppler velocimeter 0.2 0.5 512 3600

Pressure sensor 1.0 0.5 512 3600

Salinity sensor 1.0 0.5 512 3600

Optical backscatter sensor 0.2 0.5 512 3600


Acoustic Doppler

current profiler

2.0e10.0 every 1.0 0.25 40 240

Temperature sensor 1.0 0.25 40 240

MiniPROBE 4 Acoustic Doppler

current profiler

1.0e4.0 every 0.5 0.25 40 240

Temperature sensor 0.2 0.25 40 240


Pressure sensor 0.2 0.5 512 3600



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of the trade winds across the Hawaiian Islands. This period is

characterized by consistent northeast trade winds (prevalent

80e95% of the time; Western Region Climate Center, 2006)

and small trade wind waves (Fig. 2). All across the fringing

reef, the currents were primarily alongshore and their magni-

tude was strongly influenced by the tides, similar to the obser-

vations by Storlazzi et al. (2004) over the fringing reef offsouthern Molokai, Hawaii. In general, the currents show less

coherence with the tides 0.2 mab over the fore reef and

1.0 mab on the reef flat than 6.0 mab over the fore reef, likely

due to hydrodynamic roughness of the seafloor. The mean dif-

ference in direction between near-surface currents and near-

bed currents over the fore reef was 32.8 33.0, while the

mean difference in direction between currents over the fore

reef and those on the reef flat was 65.2 53.1.

During this time period the turbidity was always greatest

0.2 mab and lowest 6.0 mab over the fore reef; turbidity

1.0 mab on the reef flat was always greater than 6.0 mab

over the fore reef. Turbidity 6.0 mab over the fore reef in-

creased with offshore-directed currents and falling tides, sim-ilar to the observations made off the south Molokai fringing

reef (Storlazzi et al., 2004). At 0.2 mab on fore reef however,

turbidity increased with increasing tidal elevation, opposite

what was seen at 6.0 mab; turbidity at 0.2 mab over the fore

also increased with wave height but was not correlated with

the currents. Turbidity at 1.0 mab over the reef flat increased

with higher tidal elevations, wind speeds, wave heights and

both offshore-directed and alongshore currents downwind to

the southwest;Ogston et al. (2004)made similar observations

of turbidity over the south Molokai reef flat. All across the

reef, water temperature was positively correlated with falling

tides and offshore-directed currents; while all significant, thesecorrelations explained more of the variance high up in the

water column over the fore reef than closer to the bed or on

the reef flat.

Two distinct events during this time period show the typical

modification of these general low-frequency trends. During the

period of Year Day 2002 (YD) 266e271 and YD 287e291

(Fig. 2), propagation of atmospheric fronts to the north of

the study area caused the northeast trade winds to weaken

and be replaced by southerly winds. These periods were

marked by higher variance in the subtidal flow field all across

the reef and slightly elevated turbidity over the fore reef. In

contrast, turbidity over the reef flat was slightly less during

these events than otherwise in the summer/early fall period.

A 5-day time series of sub-tidal (>36 h) low-pass filtered

alongshore wind velocity, water level, alongshore current

velocity, turbidity and water temperature during a period of

relatively consistent wave forcing displays the influence of

wind on fore reef and reef flat processes (Fig. 3). As the trade

winds began to wane, the alongshore wind velocity to the

southwest decreased below 6 m/s, the water level began to

drop, and the alongshore currents shifted from southwestward

(downwind) to northeastward flow. The currents over the fore

reef appeared to responding more rapidly to the reduction in

alongshore winds than those over the reef flat, which did not

change substantially until YD 288. Concurrently with this

decrease in alongshore flow to the southwest, turbidity de-

creased and temperature began to increase over both the fore

reef and reef flat. These data suggest that some type of

large-scale relaxation may occur when the trade winds de-

crease in strength, with the warmer, less turbid water in the

channels between Maui, Molokai, Lanai and Kahoolawe flow-

ing northwestward out through the Pailolo Channel. Theseflows may be due to the passage of an island-trapped wave

(ITW), as observed byFlament and Lumpkin (1996)and mod-

eled byMerrifield et al. (2002).

Roughly once per day when the trade winds blew consis-

tently and wave heights were small, a very rapid (typically

0.02 1/s) and vertical gradients in

turbidity. At the time of the rapid change in water temperature,

the flow in the water column would always rapidly switch

direction, with onshore near-surface flow changing to offshore

near-surface flow and near-bed offshore flow changing to

onshore near-bed flow, or vice versa; these typically coincided

with changes in the vertical turbidity gradient, often resulting

in inverted turbidity profiles (higher turbidity near the surface

than near the seabed). These rapid changes in water tempera-

ture and flow directions were followed by cyclical variations inflow, temperature and turbidity. A rapid warming typically oc-

curred when near-surface waters moved onshore and near-bed

waters moved offshore while rapid cooling typically occurred

when near-surface waters moved offshore and near-bed waters

moved onshore. These features showed similar structure to in-

ternal tidal bores observed elsewhere on coral reefs (Wolanski

and Delesalle, 1995; Leichter et al., 2003).

4.2. Winter

During the winter months, the Hawaiian Islands are located

to the north of the heart of the trade winds. The trades still

blow across the islands much of the wintertime, though less

regularly (50e80% of the time; Western Region Climate

Center, 2006). Major storms occur most frequently during

this period, often bringing heavy rain, strong winds and large

waves (Fig. 5). Cold fronts and low-pressure systems, known

locally as Kona storms, bring heavy rains, often accompanied

by strong winds and large waves from directions other than

that of the northeast trade winds. While the seafloor sediment

is a well-sorted, coarsely-skewed clean carbonate sand at both

instrument locations and the coarse-grained (>62.5 mm) sedi-

ment collected in both sediment traps during this period was

relatively clean whitish carbonate sand (>74% carbonate),

the fine-grained component (


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260 265 270 275 280 285 290 295 300

260 265 270 275 280 285 290 295 300

260 265 270 275 280 285 290 295 300

260 265 270 275 280 285 290 295 300

260 265 270 275 280 285 290 295 300

260 265 270 275 280 285 290 295 300

260 265 270 275 280 285 290 295 300

260 265 270 275 280 285 290 295 300

260 265 270 275 280 285 290 295 300

-0.2

0

0.2

0

15

-0.2

0

0.2

0

15

-10

0

10

-0.5

0

0.5

-2

0

2

-0.2

0

0.2

0

50

100

30

30

C

urrent

[m/s]

Turbidity

[NTU]

Year Day 2002

Current

[m/s]

Turbidity

[NTU]

Wind

[m/s]

Tide

[m]

Wave

[m]

Current

[m/s]

Turbidity

[NTU]

Fore Reef 6.0 mab

Fore Reef 6.0 mab

Fore Reef 0.2 mab

Fore Reef 0.2 mab

Reef Flat 1.0 mab

Reef Flat 1.0 mab

a

b

c

d

e

f

g

h

i

Fig. 2. Example of the variability in hourly forcing and the water columns response during the quiescent early fall months. (a) Tide. (b) Wave height (gray) and

magnitude and direction (black). (c) Wind speed (gray) and magnitude and direction (black; NCDC, 2004). (d) Current speed and direction 6.0 mab on the fore

reef. (e) Turbidity 6.0 mab on the fore reef. (f) Current speed and direction 0.2 mab on the fore reef. (g) Turbidity 0.2 mab on the fore reef. (h) Current speed and

direction 1.0 mab on the reef flat. (i) Turbidity 2.0 mab on the reef flat. Note the varying scales on the y-axes.



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primarily (>68%) reddish terrigenous mud and contained

numerous (>4%) organic particles.

In contrast to the more quiescent late summer and early fall

months (Fig. 2), both the currents and the turbidity all across

the fringing reef were less coherent with the tides. The mean

variation in direction between near-surface currents and

near-bed currents over the fore reef during the winter was

approximately 43% greater (47.2 46.5) than in the summer

and early fall, while the mean variation in direction between

currents over the fore reef and those on the reef flat was

approximately 26% less (48.2 49.5). When storms pass

through the Hawaiian Islands and cause winds to blow from

a direction other than their typical northeasterly orientation,

the southwest-northeast oscillating tidal currents that charac-

terize flow over the reef are replaced by strong net flows to

the northeast. These storms also bring larger than normal

waves to the study area, and turbidity all across the fringing

reef during large wave events was more coherent with wave

285

-0.08

-0.04

0

0.04

0.08

AlongshoreCurrent

[m/s]

285

2

4

6

8

Turbidity

[NTU]

26.25

26.50

26.75

Temperature

[C]

285.5 286 286.5 287 287.5 288 288.5 289 289.5 290

285.5 286 286.5 287 287.5 288 288.5 289 289.5 290

285 285.5 286 286.5 287 287.5 288 288.5 289 289.5 290

Year Day 2002

23

24

25

26

to southwest

to northeast

285 285.5 286 286.5 287 287.5 288 288.5 289 289.5 290-0.05

0

0.05

WaterLevel

[m]

285-8

-4

0

4

8

A

longshoreWind

[m/s]

285.5 286 286.5 287 287.5 288 288.5 289 289.5 290

to southwest

to northeast

a

b

c

d

e

Fig. 3. Thirty-six hour low-pass filtered time series data showing the influence of alongshore wind forcing on sub-tidal water level, flow and water column prop-

erties over the fore reef. (a) Alongshore wind stress (NCDC, 2004). (b) Water depth. (c) Alongshore currents 2.0 mab (black) and 8.0 mab (gray) over the fore reefand 1.0 mab on the reef flat (dashed). (d) Turbidity 2.0 mab (black) and 8.0 mab (gray) over the fore reef and 1.0 mab on the reef flat (dashed; values on second

y-axis). (e) Water temperature 1.0 mab over the fore reef (black) and 1.0 mab on the reef flat (dashed).



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height than with the tides. Turbidity was greater 0.2 mab on

the fore reef than 6.0 mab over the fore reef or 1.0 mab on

the reef flat during this time period. While turbidity over the

fore reef was greater during this time period than during the

more quiescent late summer and early fall months, turbidity

over the reef flat, while reaching greater maximum values

than during the late summer and early fall months, had lower

mean and minimum values during the more energetic winter

months. The cross-shore and alongshore currents explained

more of the variance in turbidity over the fore reef during

the winter than during the summer months, but the cross-shore

currents still explained roughly twice the variance of the

alongshore currents (23e50% vs. 5e27%). Cross-shore cur-

rents over the reef flat explained almost twice the variance

in turbidity than during the summer and early fall months

(43% vs. 26%), while during the winter months the alongshore

currents were no longer significantly correlated with turbidity.

While temperature and salinity remained uncorrelated with

alongshore currents, cross-shore currents explained, on aver-

age, twice the variance in temperature and salinity than during

the summer and early fall months.

To better constrain the influence of waves on flow over the

fringing reef, two time periods with similar wind and tidal

forcing were examined (Fig. 6). Under small wave conditions

(mean 1ssignificant wave heights 0.56 0.06 m at 359)

and relatively constant wind forcing during YD 270e272, the

flow over the fore reef was primarily alongshore, with mean

flow to the northeast, and an increasingly greater onshore com-

ponent of flow closer to the bed. The greater onshore compo-

nent of flow near the bed is possibly due to wave orbital

asymmetry as the waves shoal up the fore reef or topographic

steering of the currents by the local reef morphology; it is

-0.2

-0.1

0

0.1

0.2

Tide[m]

Year Day 2002

74.5 74.55 74.6 74.65 74.7 74.75 74.8 74.85 74.9 74.95 75

74.5 74.55 74.6 74.65 74.7 74.75 74.8 74.85 74.9 74.95 75

74.5 74.55 74.6 74.65 74.7 74.75 74.8 74.85 74.9 74.95 75

-0.10

-0.05

0

0.05

0.10

OnshoreCurrent[m/s]

74.5 74.55 74.6 74.65 74.7 74.75 74.8 74.85 74.9 74.95 75

Turbidity[NTU

]

8

9

10

11

12

23.8

24.0

24.2

24.4

24.6

Tem

perature[C]

a

b

c

d

Fig. 4. Time series data showing the influence of high-frequency vertical velocity shear on flow and water column properties over the fore reef. (a) Tide. (b) Cross-

shore currents 2.0 mab (black) and 8.0 mab (gray). (c) Turbidity 2.0 mab (black) and 8.0 mab (gray). (d) Water temperature 1.0 mab. Solid lines are 4-min data;

dotted lines are 20-min low-pass filtered data. These features show similar structure to high-frequency internal bores observed on the inner shelf at many locations

around the globe.



9/16

not clear at this time which process or processes cause this on-

shore flow. On the reef flat, however, the flow was more

variable, with mean flow downwind to the southwest. Thus

there was significant cross-shore velocity shear, with the

mean flow over the fore reef heading upwind to the northeast

and flow over the reef flat heading downwind to the south.

Mean wind- and wind-driven wave flows downwind to the

south over the reef flat and flow upwind to the northeast over

360 365 370 375 380 385 390 395 400

360 365 370 375 380 385 390 395 400

360 365 370 375 380 385 390 395 400

360 365 370 375 380 385 390 395 400

360 365 370 375 380 385 390 395 400

360 365 370 375 380 385 390 395 400

360 365 370 375 380 385 390 395 400

360 365 370 375 380 385 390 395 400

360 365 370 375 380 385 390 395 400

Current

[m/s]

Turb

idity

[NTU]

Year Day 2002

Cu

rrent

[m

/s]

Turbidity

[NTU]

Wind

[m/s]

Tide

[m]

Wave

[m]

Current

[m/s]

-0.5

0

0.5

Turbidity

[NTU]

Fore Reef 6.0 mab

Fore Reef 6.0 mab

Fore Reef 0.2 mab

Fore Reef 0.2 mab

Reef Flat 1.0 mab

Reef Flat 1.0 mab

0

50

100

-0.2

0

0.2

0

15

-0.2

0

0.2

0

15

-10

0

10

-2

0

2

-0.2

0

0.2

30

30

a

b

c

d

e

f

g

h

i

Fig. 5. Example of the variability in hourly forcing and the water columns response during the energetic winter months. (a) Tide. (b) Wave height (gray) and

magnitude and direction (black). (c) Wind speed (gray) and magnitude and direction (black; NCDC, 2004). (d) Current speed and direction 6.0 mab on the

fore reef. (e) Turbidity 6.0 mab on the fore reef. (f) Current speed and direction 0.2 mab on the fore reef. (g) Turbidity 0.2 mab on the fore reef. (h) Current speed

and direction 1.0 mab on the reef flat. (i) Turbidity 2.0 mab on the reef flat. Note the varying scales on the y-axes.



10/16

the fore reef were often observed using a vessel-mounted,downward-looking ADCP in this area during the summer of

2003 (Storlazzi et al., 2006b); similar distinctions were ob-

served over reefs in the Caribbean byRoberts et al. (1980).

Under large wave conditions (mean 1s significant wave

heights 1.67 0.58 m at 308; maximum significant wave

height >2.5 m), the mean flow out over the fore reef was up-

wind to the north and had a component of offshore flow, which

increased close to the bed; variability in current direction was

also greater throughout the water column over the fore reef

during this time period. Most impressive, however, was the

strong mean offshore flow on the reef flat, with almost no

onshore flow observed 1.0 mab during this time period.

5. Discussion

Variations in flow and turbid water transport along the

fringing reef off Kahana, northwest Maui, can be categorized

by four sets of meteorologic and oceanographic conditions:

trade winds, relaxation events, large wave events and storms.

Trade wind periods, which occurred 71.1% of the year, are

defined by strong (>5 m/s) alongshore winds to the southwest

and small (1 m) waves

and weaker (


11/16

reef, the phasing of the currents and turbidity was such that the

mean turbidity flux was oriented almost directly offshore. The

near-surface turbidity flux over the fore reef, however, was ori-

ented downwind to the southwest, showing more coherent

phasing between the ebbing tidal currents and turbidity.

Over the reef flat, the mean turbidity was much greater than

out over the fore reef and was correlated to tidal elevation,

similar to the observations byOgston et al. (2004)andPresto

AlongshoreCurrentVelocity[m

/s]

Onshore Current Velocity

[m/s]

Trade Wind

-0.10 0 0.10

-0.10

-0.05

0.05

0.10

0.05-0.05

-0.10 0 0.100.05-0.05

-0.10 0 0.100.05-0.05

-0.10 0 0.100.05-0.05

AlongshoreFlux[NTU-m/s]

Flow

(Current)

Flux

(Turbidity * Current)

Onshore Flux

[NTU-m/s]

0

-0.10

-0.05

0.05

0.10

0

-0.10

-0.05

0.05

0.10

0

-0.10

-0.05

0.05

0.10

0

Trade Wind

-2.0 0 2.0

-2.0

-1.0

1.0

2.0

1.0-1.0

-2.0 0 2.01.0-1.0

-2.0 0 2.01.0-1.0

-2.0 0 2.01.0-1.0

0

-2.0

-1.0

1.0

2.0

0

-2.0

-1.0

1.0

2.0

0

-2.0

-1.0

1.0

2.0

0

Relaxation

Large Waves

Storm

Relaxation

Large Waves

Storm

Reef Flat, 1.0 mab

Fore Reef, 0.2 mab

Fore Reef, 2.0 mab

Fore Reef, 8.0 mab

Fig. 7. Variability in mean flow (left column) and turbidity flux (right column), calculated as the product of the mean current velocity and mean turbidity, for

different location across the reef and at different depths, under four combinations of meteorologic and oceanographic forcing. Top to bottom: Trade winds, defined

by strong (>5 m/s) alongshore winds to the southwest and small (1 m) waves and weaker (


12/16

et al. (2006) over the reef flat off south Molokai. The data

presented here suggest that the reef flat here is depth-limited,

and greater trade wind-driven waves and wind-driven currents,

and thus combined wave-current shear stresses, can develop at

higher tidal elevations. Greater shear stresses appear to sus-

pend greater volumes of seafloor sediment, which are then

transported downwind by the trade wind-driven currents.Water temperatures were more variable during trade wind

periods, and the rapid changes in water temperature that ap-

peared to be internal bores were consistently observed. Time

series measurements of thermal stratification were not made

by the instruments described in this study. Spatial surveys

made in the study area during 2003 (Storlazzi et al., 2006b),

however, showed thermal stratification, which is necessary

for the formation and propagation of internal motions, to be

greatest during periods of small waves and consistent trade

wind forcing. While it is not clear if these internal motions

observed over the fore reef are related to the deep internal

tide generated by the interaction of the barotropic tide with

the Hawaiian Ridge (Pinkel et al., 2000), similar internal boreshave been shown to cause upwelling around islands (Wolanski

and Delesalle, 1995) and episodically transport deep, subther-

mocline nutrients from offshore up onto the coral reefs in the

Florida Keys (Leichter et al., 2003).

5.2. Relaxation

Unlike the nearly symmetric alongshore currents observed

over the fore reef during trade wind periods, the currents dur-

ing relaxation events were skewed to the northeast, sometimes

to the point of having no flow to the southwest, even during the

falling tide. Over the fore reef, mean flow during relaxationevents was to the northeast throughout the water column

(Fig. 7). On the reef flat, however, mean flow was relatively

weak and oriented obliquely onshore to the southeast. This

southeast mean flow on the reef flat, without concurrent

wind forcing in the same orientation, suggests that these

mean flows are driven by some other process, likely wave-

orbital asymmetry as the waves shoal over the reef flat. The

water generally warmed during these periods, likely due to

the advection of warmer water from inside the Maui Nui com-

plex northeastward past the study site.

Turbidity varied across the reef and was generally higher

during falling tides, although it showed much less coherence

with the tides than when the trade winds were steady. Turbidity

just above the seafloor over the fore reef was slightly higher

during relaxation events; further up in the water column over

the fore reef and over the reef flat it was lower than during con-

sistent trade winds. The lower turbidity on the reef flat during

relaxation events further emphasizes the importance of the

interaction between trade wind waves and currents on sediment

resuspension on the depth-limited reef flat. The higher turbidity

close to the bed over the fore reef, however, may be due to the

slightly higher waves that often impact the study area when the

low-pressure systems that cause the trade winds to wane pass

the island chain. While flux high up in the water column

over the fore reef and on the reef flat were aligned with the

flow and headed to the north and southeast, respectively, the

flux close to the bed was offshore to the west, showing signif-

icant correlation between offshore flow and high turbidity.

5.3. Large waves

During large wave events the currents were symmetric andoriented predominantly alongshore (Figs. 6 and 7). Close to

the bed over the fore reef and up on the reef flat, the mean

flow was oriented almost directly offshore, showing little

influence of the tides. The offshore-directed mean flow over

the reef flat compared to the downwind orientation of mean

flow under trade winds and small waves suggests some type

of offshore-directed, near-bed return flow balancing the

wave-driven onshore surface flow up onto the reef by the

larger waves, similar to the observations made over the fring-

ing reef off south Molokai (Storlazzi et al., 2004) and modeled

by Gourlay (1996). This return flow might also explain the

greater magnitude of the mean flows higher up in the water

column over the fore reef, which is approximately two timesgreater than under small wave conditions. The phasing of

flow and turbidity were such that mean flux is oriented

offshore, with the near-bed flux over the fore reef and up on

the reef flat oriented almost directly offshore. The mean flux

higher up in the water column over the fore reef, while ori-

ented offshore, shows greater coherence between alongshore

flow and high turbidity.

5.4. Storms

When storms pass through the study area, they block the

normal trade winds, causing strong winds from other orienta-tions (usually the south), and bring large waves, resulting in

flow that has characteristics that are a mix of relaxation events

and large waves (Fig. 7). Similar to relaxation events, the cur-

rents up in the water column over the fore reef were asymmet-

ric to the northwest and alongshore flow to the southwest, even

during falling tides, was rarely observed. Flows close to the

bed over the fore reef and on the reef flat were directed off-

shore, however, and were similar to those observed under large

wave conditions; mean flux also showed similar orientations.

5.5. Implications of transport patterns to reef processes

The spatial, both horizontally across the reef and vertically

through the water column, and temporal differences in the

magnitude and direction of currents and turbidity flux demon-

strate the complexity of flow and the transport of turbid water

along a relatively geometrically simple fringing reef. At

a given location on the reef, there is substantial vertical vari-

ability in current orientation, with flow directions and the

resulting turbidity flux often varying by more than 90 in the

relatively shallow (w10 m) water column (Fig. 7). Thus,

material distributed throughout the water column, including

the particles in suspension causing the turbidity (e.g. sediment

or larvae) and/or dissolved nutrients and contaminants, under

constant oceanographic and meteorologic forcing, could be



13/16

transported in different directions. Furthermore, under con-

stant forcing, the transport of material at the same height

above the seafloor would be different at different locations

across the reef depending on the water depth, similar to the

observations made byRoberts et al. (1992) in the Caribbean.

The data presented here, however, suggest that off western

Maui the delineations made byRoberts et al. (1992)betweenwave-dominated and current-dominated regions of a fringing

reef are not just based on location across the reef, but vary

temporally and vertically in the water column.

The temporally- and spatially-varying nature of flow and

flux has implications for both biologic and geologic processes.

Kolinski and Cox (2003)show that 71% of broadcast spawn-

ing corals in Hawaii display peak gamete or planula release

during the summer (June-September) when the flows over

the fringing reef off northwest Maui are dominated by trade

wind conditions. This would imply that larvae spawned from

the reefs in the study area would likely be carried offshore

and downwind by the prevailing currents, similar to the obser-

vations made byStorlazzi et al. (2006a)during the 2003 sum-mer spawning season. While this would not result in seeding

of the natal reef, it would circulate larvae within the islands

of Maui, Molokai, and Lanai, increasing the supply and diver-

sity of larvae to the reefs on adjacent islands downstream from

the study area. The summer spawning period is also character-

ized by the high-frequency internal bores, which may help to

bring settling larvae back onshore into suitable water depths or

habitat to settle and recruit. However, these same bores also

have the potential to advect nutrient or contaminant-laden

deep injection well water (Soicher and Peterson, 1996; West

Maui Watershed Management Project, 1996; Dollar and

Andrews, 1997) that has percolated out of the island at depthup onto the reefs during this biologically important period.

While the largest delivery of sediment and potential contam-

inants by fluvial discharge or overland flow is greatest during

the winter when rainfall is highest, the processes of transport

over the fringing reef off Kahana decrease the impacts of these

inputs. The large wave stresses and strong offshore flow over

the reef flat during winter storms and large wave events would

resuspend the relatively fine-grained, slowly-settling sediment

deposited on the reef flat from terrestrial sources and transport

it offshore (Fig. 7), possibly in a nepheloid layer similar to that

observed by Wolanski et al. (2003). These same large wave

stresses and the strong offshore flows throughout the water

column over the fore reef would limit the deposition and thus

the impact of the fine-grained terrestrial sediment on the corals

on the fore reef by advecting it quickly offshore beyond the

fore reef system.

Further evidence of offshore transport of fine-grained ter-

restrial sediment derives from sediment traps attached to

both the fore reef and reef flat instrument packages during

the last deployment. The high percentage (>68%) of the

fine-grained material being terrigenous in origin found in

both traps (as discussed in Section 4.2) but not observed on

the surrounding seabed suggest that, while the energetics of

the environment are generally too great to allow for the

long-term deposition of terrestrial fine-grained silts and clays

on the seabed, large quantities of fine-grained terrigenous

sediment move through the study area. Because sediment traps

generally under-sample slowly-settling, fine-grained material

in energetic environments (e.g.Gardner et al., 1983), we inter-

pret the high percentage of fine-grained terrestrial material in

both the reef flat and fore reef traps to mean that there are

large quantities of terrestrial fine-grained sediment movingthrough the system. Thus, while not observed on the seabed

during low-energy conditions or incorporated in the geologic

record, this terrigenous sediment is advected over the reef,

potentially decreasing photosynthetically-available radiation

(PAR) and possibly desorbing nutrients (De Carlo and Dollar,

1997) and/or contaminants.

The transport of turbid water over the fringing reef is highly

seasonal. Over the course of a year, the majority of the turbid-

ity flux occurred during the trade wind-dominated summer

period (Fig. 8). While turbidity is greatest during large wave

events and storms, the low frequency of these conditions

results in lower net flux than the lower-energy, but much

more frequent, trade wind conditions. The moderately highturbidity levels on the reef flat during the spring and summer

when trade winds dominate the forcing is likely caused by the

resuspension of sediment deposited on the inner reef flat by

fluvial discharge that generally occurs in the winter; this sed-

iment is then transported downwind to the southwest, with

limited exchange between the reef flat and fore reef. The daily

increases and decreases in wind-driven current and wind-

driven wave stresses, in conjunction with the phasing of these

stresses and tidal elevation, appears to result in some sediment

being resuspended and then settling out each day (as inferred

from rhythmic turbidity levels) on the reef flat as it is trans-

ported downwind along the reef flat. This repetitive resuspen-sion and deposition likely increases the relative impact of this

sediment on biologic processes, since a given sediment parti-

cle might be resuspended, causing turbidity and decreasing

PAR, then re-deposited on a coral each day. This probably

causes physiologic stress or at least a decrease in usable

energy as the coral receives reduced PAR and diverts energy

for mucus production to slough off the settling sediment

(Fabricius, 2005).

Fine-grained terrestrial material, which was collected in

both the reef flat and fore reef sediment traps, is of concern

because its dark color and low settling velocity means that it

tends to stay in suspension for long periods of time and thus

block the incident PAR needed by the symbiotic photosyn-

thetic algae in the corals. Resuspension of coarser bed material

on the reef flat appears to be limited to periods of concurrent

high tides, large waves and strong trade winds when combined

wave-current shear stresses are high; the net transport of this

material on the reef flat is limited due to its high settling ve-

locity and the shorter period of resuspension. Near-bed flows

over the fore reef under all sets of conditions were offshore;

this offshore-directed bottom flow may be balancing wave-

and wind-driven onshore surface flow in a thin surface layer

and may represent the mechanism by which the reef sheds

material generated by bioerosion and mechanical abrasion of

corals and coralline algae.



14/16

Overall, under these four combinations of meteorologic and

atmospheric forcing, velocity shear, both vertically through the

water column and horizontally across the reef, was observed.

This shear results in different magnitudes and directions of

flow and the transport of turbid water, highlighting the ex-

tremely complex nature of flow over a relatively geometrically

simple fringing reef. These findings suggest that sampling pro-

tocols and field experiments need much greater spatial and tem-

poral resolution to reflect the true range of variability in flow,

water column properties, and the transport of turbid water

over a fringing reef than along less complex, sandy shorelines

where flow features are often coherent over length scales an or-

der of magnitude greater. Furthermore, while the relative

contribution of these different combinations of meteorologic

and atmospheric forcing may vary from reef to reef, since the

majority of the worlds coral reefs lie within the northern and

southern hemispheres trade wind belts (Spalding et al., 2001),

the general mechanisms and patterns described here likely char-

acterize a wide range of fringing reefs around the world.

6. Conclusions

The data presented here detail how flow and water column

properties vary over a fringing reef over time scales ranging

from minutes to seasons and are driven by waves, winds, tides

and internal bores. Four sets of meteorologic and oceanographic

Alongsho

reFlux[km-NTU]

Onshore Flux [km-NTU]

Fore Reef: 8.0 mab

-70 70-70

-35

35

70

35-35 -70 7035-35

0

-70

-35

35

70

0

-70

-35

35

70

0

-70

-35

35

70

0

Fore Reef: 2.0 mab

Fore Reef: 0.2 mab Reef Flat: 1.0 mab

Trade Winds

(71.1%)

Relaxation

(16.5%)

Storm

(2.3%)

Large Waves

(10.1%)

a b

dc

-70 7035-35 -70 7035-35

0 0

00

Fig. 8. Variability in net turbidity flux, calculated as the cumulative product of the current velocity and turbidity over the duration of each of the four combinations

of meteorologic and oceanographic forcing defined inFig. 7, for different location across the reef and at different depths. (a) Fore reef at 8.0 mab. (b) Fore reef at

2.0 mab. (c) Fore reef at 0.2 mab. (d) Reef flat at 1.0 mab. Positive alongshore is defined upwind to the northeast and the units are: km-NTU. The percentages

represent the frequency distribution of these different sets of conditions over the 12-month period between February 2002 and February 2003, when the data

coverage was greatest. Note that net turbidity flux is almost always offshore on the fore reef, especially close to the seafloor, and net flux both on the reef flat

and over the fore reef is dominated by the persistent trade wind forcing.



15/16

conditions were defined that control the spatial and temporal

variability in flow and transport over the fore reef and reef flat

at Kahana, northwest Maui. Throughout the period of study,

near-surface current directions over the fore reef varied on aver-

age by more than 41 from those near the seafloor, and the ori-

entation of the currents over the reef flat differed on average

by more than 65

from those observed over the fore reef. Thelate spring, summer, and early fall were characterized by low

wave energy and relative consistent trade wind forcing. During

this time, which encompasses the coral spawning season, there

was downwind transport on the fringing reef, punctuated by up-

wind flow during periods of weak winds or reversals in wind di-

rection. Internal bores were frequently observed during this

time. During the energetic winter months, turbidity was much

greater and flow throughout most of the water column, both

over the fore reef and on the reef flat, was oriented offshore.

While turbidity is greatest during large wave events and storms,

the low frequency of these conditions results in lower annual

flux on the fringing reef than the lower-energy, but much more

frequent, trade wind conditions that typify the late spring, sum-mer, and early fall. These observations demonstrate the high

spatial variability in flow and the transport of turbid water across

a relatively geometrically-simple fringing reef. Only by under-

standing the natural variability of these parameters over the

range of seasons that characterize an area can one design sam-

pling schemes that will be representative of the annual budget

of the system, or conversely, put the limited measurements typ-

ically made in the context of the total variability of the system.

Acknowledgments

This work was carried out as part of the U.S. GeologicalSurveys Coral Reef Project as part of an effort in the United

States and its trust territories to better understand the effects

of geologic processes on coral reef systems. We would like to

thank Project Chief Mike Field, whose support made it possible

to conduct such an extensive field experiment. Eric Brown

(NPS), John Gorman, Jim Luecke and Fred Putnam (Maui

Ocean Center) went out of their way to help us carry out this ex-

periment and for that we owe them great thanks. We would also

like to thank Jodi Eshleman (USGS), Kurt Rosenberger (USGS),

and Eric De Carlo (UH), who contributed numerous excellent

suggestions and a timely review of our work. Use of trademark

names does not imply USGS endorsement of products.

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Documents

Storlazzi 2008 Estuarine, Coastal and Shelf Science